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1 – 10 of over 15000Qiuchen Zhao, Xue Li, Junchao Hu, Yuehui Jiang, Kun Yang and Qingyuan Wang
The purpose of this paper is to determine the ultra-high cycle fatigue behavior and ultra-slow crack propagation behavior of selective laser melting (SLM) AlSi7Mg alloy under…
Abstract
Purpose
The purpose of this paper is to determine the ultra-high cycle fatigue behavior and ultra-slow crack propagation behavior of selective laser melting (SLM) AlSi7Mg alloy under as-built conditions.
Design/methodology/approach
Constant amplitude and two-step variable amplitude fatigue tests were carried out using ultrasonic fatigue equipment. The fracture surface of the failure specimen was quantitatively analyzed by scanning electron microscope (SEM).
Findings
The results show that the competition of surface and interior crack initiation modes leads to a duplex S–N curve. Both manufacturing defects (such as the lack of fusion) and inclusions can act as initially fatal fatigue microcracks, and the fatigue sensitivity level decreases with the location, size and type of the maximum defects.
Originality/value
The research results play a certain role in understanding the ultra-high cycle fatigue behavior of additive manufacturing aluminum alloys. It can provide reference for improving the process parameters of SLM technology.
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This paper aims to predict fatigue life and fatigue limit of fiber-reinforced ceramic-matrix composites (CMCs) with different fiber preforms, i.e. unidirectional, cross-ply, 2D-…
Abstract
Purpose
This paper aims to predict fatigue life and fatigue limit of fiber-reinforced ceramic-matrix composites (CMCs) with different fiber preforms, i.e. unidirectional, cross-ply, 2D-, 2.5D- and 3D-woven, at room and elevated temperatures.
Design/methodology/approach
Under cyclic loading, matrix multicracking and interface debonding occur upon first loading to fatigue peak stress, and the interface wear appears with increasing cycle number, leading to degradation of the interface shear stress and fibers strength. The relationships between fibers fracture, cycle number, fatigue peak stress and interface wear damage mechanism have been established based on the global load sharing (GLS) criterion. The evolution of fibers broken fraction versus cycle number curves of fiber-reinforced CMCs at room and elevated temperatures have been obtained.
Findings
The predicted fatigue life S–N curve can be divided into two regions, i.e. the Region I controlled by the degradation of interface shear stress and fibers strength and the Region II controlled by the degradation of fibers strength.
Practical/implications
The proposed approach can be used to predict the fatigue life and fatigue limit of unidirectional, cross-ply, 2D-, 2.5D- and 3D-woven CMCs under cyclic loading.
Originality/value
The fatigue damage mechanisms and fibers failure model were combined together to predict the fatigue life and fatigue limit of fiber-reinforced CMCs with different fiber preforms.
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Koji Takahashi, Hideki Okada and Kotoji Ando
The purpose of this paper is to clarify the effects of shot peening (SP) on the torsional fatigue limit of high‐strength steel specimens containing an artificial small defect.
Abstract
Purpose
The purpose of this paper is to clarify the effects of shot peening (SP) on the torsional fatigue limit of high‐strength steel specimens containing an artificial small defect.
Design/methodology/approach
Specimens containing a drilled hole 0.1‐0.4 mm deep or a semi‐circular slit 0.15 or 0.3 mm deep were subjected to SP. Torsional fatigue tests were then carried out.
Findings
The torsional fatigue limits of specimens containing a drilled hole and those with a semi‐circular slit were increased 25‐64 per cent and 156‐186 per cent by SP, respectively. The torsional fatigue limits of the specimens subjected to SP and containing a drilled hole less than 0.1 mm in depth or a semi‐circular slit less than 0.15 mm in depth were almost equal to those of SP specimens without a defect. Based on these results, it can be concluded that a drilled hole less than 0.1 mm in depth and a semi‐circular slit less than 0.15 mm in depth could be rendered harmless by SP.
Practical implications
The proposed method can be applied to mechanical parts subjected to cyclic torsion, such as coil springs, crank shafts and drive shafts.
Originality/value
This is the first paper to investigate the torsional fatigue limits after SP in materials containing a surface defect. In this paper, the effect of SP on the torsional fatigue limit having a surface defect is investigated.
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Wyman Zhuang, Qianchu Liu and Cathy Smith
One of the challenges in the prediction of fatigue crack growth is to identify representative initial flaws and defects that can cause fatigue crack initiation and subsequent…
Abstract
Purpose
One of the challenges in the prediction of fatigue crack growth is to identify representative initial flaws and defects that can cause fatigue crack initiation and subsequent crack growth. Representative initial flaws identified from this experimental study provided an essential input for the fatigue life assessment programme of the PC-9/A training aircraft currently in service. The paper aims to discuss these issues.
Design/methodology/approach
This paper addresses this challenge with a critical literature review and experimental assessment of initial flaw types that may cause fatigue crack initiation, by fatigue testing and fractography analysis using optical microscope and scanning electron microscopy (SEM).
Findings
With a focus on aluminium alloy (AA) 2024-T3 thin sheet, the results cover various discontinuities from microstructural constituent particles inherent from the material process to macrostructural defects and surface discontinuities (such as burrs and machining marks) introduced during the production of airframes. It was found that most fatigue cracks originated from the bore surface discontinuities of rivet holes in the PC-9 vertical stabiliser thin panels rather than microstructural material defects of AA2024-T3 inherent from the material process.
Research limitations/implications
The experimental study has found that quantifying fatigue initial flaw sizes which resulted from poorly finished fastener holes with arbitrary discontinuities at the surface is a challenging topic. This topic is under the current investigation using a statistics based analysis of initial flaws in the prediction of fatigue crack growth.
Practical implications
The results obtained from this experimental study provided an essential input for the empennage and aft fuselage recertification and life assessment programme for the PC-9/A training aircraft currently in service.
Originality/value
This experimental study examined AA2024-T3 thin skin panels from two different PC-9/A aircraft. The post-test failure analysis using optical microscope and SEM found that machining defects dominate fatigue crack initiation that can result in subsequent crack propagation.
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Keiji Houjou, Koji Takahashi and Kotoji Ando
The purpose of this study was to investigate the effect of overload (bending moment with plastic deformation: Mp) on three point bending specimen at the fatigue limit of…
Abstract
Purpose
The purpose of this study was to investigate the effect of overload (bending moment with plastic deformation: Mp) on three point bending specimen at the fatigue limit of high-tensile-strength steel containing a crack in the stress concentration zone.
Design/methodology/approach
An artificial semi-circular slit was introduced and Mp was applied after which bending fatigue tests were carried out.
Findings
The relationship between the level of Mp and the fatigue limit (σw) was proportional; the fatigue limits of specimens containing 0.2- and 0.3-mm-deep slits are improved by the Mp process as much as twice the original values; the slit size that can be rendered harmless by the Mp process is a=0.05 mm in depth; and all non-propagating cracks appeared around the artificial slit.
Originality/value
Very few studies have been conducted on the fatigue limit of materials containing crack-like surface defects after overload in the stress concentration zone. This study elucidated the effect of Mp on the fatigue limit.
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Jun Yasuda, Koji Takahashi and Hideki Okada
The purpose of this study is to clarify the influence of stress ratio (R) on the effects of shot peening (SP) on the fatigue limit of high-strength steel containing an artificial…
Abstract
Purpose
The purpose of this study is to clarify the influence of stress ratio (R) on the effects of shot peening (SP) on the fatigue limit of high-strength steel containing an artificial small defect.
Design/methodology/approach
SP was subjected on the specimens with a semi-circular slit with a depth of a=0.1, 0.2 and 0.3 mm. Then, bending fatigue tests were carried out under R=0.4.
Findings
The fatigue limits of specimens with a semi-circular slit were improved by SP under R=0.4. The fatigue limits of the SP specimens with a semi-circular slit under a=0.2 mm fractured outside the slit, and they had considerably high fatigue limits equal to specimens without a slit. Therefore, a semi-circular slit with a depth of under a=0.2 mm could be rendered harmless by SP under R=0.4. Compared to the results of R=0, the increasing ratios of fatigue limits under R=0.4 were lower than those under R=0. However, the size of semi-circular slit that could be rendered harmless by SP was same. In addition, it was found that whether the semi-circular slit is rendered harmless or not is decided by the relationship between the stress intensity factor range of semi-circular cracks and the threshold stress intensity factor regardless of stress ratio.
Practical implications
The proposed method can be applied to mechanical parts used in vehicles, aircraft and trains.
Originality/value
This is the first paper to investigate the fatigue limits after SP in materials containing a surface defect under positive stress ratio. In this study, the authors investigated the influence of stress ratio on the effects of SP on the fatigue limit containing a surface defects.
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Xiaoguang Sun, Xuexu Xu, Zihan Wang and Zhiyong Liu
The purpose of this paper is to determine the corrosion fatigue behavior and mechanism of 6005A aluminum alloy and welded joint.
Abstract
Purpose
The purpose of this paper is to determine the corrosion fatigue behavior and mechanism of 6005A aluminum alloy and welded joint.
Design/methodology/approach
Electron back-scattered diffraction (EBSD) were adopted to characterize the microstructure of 6005A aluminum alloy and welded joint. Through potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and corrosion fatigue experiments, the corrosion fatigue behavior and mechanism of 6005A aluminum alloy base metal and welded joint were studied.
Findings
The results show that the corrosion fatigue crack initiation of 6005A aluminum alloy base metal and welded joint is mainly caused by the preferential anodic dissolution and hydrogen concentration in the areas with inclusions and welding defects.
Originality/value
The research is an originality study on the corrosion fatigue behavior and mechanism of 6005A aluminum alloy and welded joint.
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Xingyang Chen, Linlin Ma, Haoping Xie, Fengting Zhao, Yufeng Ye and Lin Zhang
The purpose of this paper is to present a crack initiation mechanism of the external hydrogen effect on type 304 stainless steel, as well as on fatigue crack propagation in the…
Abstract
Purpose
The purpose of this paper is to present a crack initiation mechanism of the external hydrogen effect on type 304 stainless steel, as well as on fatigue crack propagation in the presence of hydrogen gas.
Design/methodology/approach
The effects of external hydrogen on hydrogen-assisted crack initiation in type 304 stainless steel were discussed by performing fatigue crack growth rate and fatigue life tests in 5 MPa argon and hydrogen.
Findings
Hydrogen can reduce the incubation period of fatigue crack initiation of smooth fatigue specimens and greatly promote the fatigue crack growth rate during the subsequent fatigue cycle. During the fatigue cycle, hydrogen invades into matrix through the intrusion and extrusion and segregates at the boundaries of α′ martensite and austenite. As the fatigue cycle increased, hydrogen-induced cracks would initiate along the slip bands. The crack initiation progress would greatly accelerate in the presence of hydrogen.
Originality/value
To the best of the authors’ knowledge, this paper is an original work carried out by the authors on the hydrogen environment embrittlement of type 304 stainless steel. The effects of external hydrogen and argon were compared to provide understanding on the hydrogen-assisted crack initiation behaviors during cycle loading.
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Sergio Baragetti, Riccardo Gerosa and Francesco Villa
7075-T6 is the most widespread structural aluminium alloy due to its high mechanical strength. However, use of this alloy in critical aeronautic, maritime, and automotive sectors…
Abstract
Purpose
7075-T6 is the most widespread structural aluminium alloy due to its high mechanical strength. However, use of this alloy in critical aeronautic, maritime, and automotive sectors is limited by the susceptibility of T6 treatment to cracking and pitting corrosion. To improve fatigue behaviour in aggressive environments, several authors have proposed the use of different coatings to protect the substrate. Studies have investigated the application of thin hard coatings on light alloys by physical vapour deposition (PVD). Different contributions of residual stresses, thermal modification of the substrate, and mechanical interaction between the coating and aluminium substrate were investigated. The purpose of this paper is to investigate the rotating bending fatigue behaviour (R=−1) of 7075-T6 PVD diamond-like carbon (DLC)-coated specimens in air and in a corrosive environment. Tests were conducted at different applied stresses. Scanning electron micrographs of the fracture surface are provided to investigate the influences of mechanical and environmental driving forces on the failure mechanism.
Design/methodology/approach
The paper conducted an experimental study of the fatigue resistance of DLC coatings on a 7075-T6 substrate for corrosion protection at long and short fatigue lives, which includes rotating bending fatigue tests, step-loading fatigue test procedure, tests in aggressive environment (methanol), tests at high and low fatigue lives, analysis of the fracture surface, and analysis of the driving forces.
Findings
Tests performed in air showed that the coating anticipates crack nucleation for high applied loads, whereas for lower loads, the difference among fatigue curves decreases. This result is very interesting from an industrial standpoint because the obtained material shows improved corrosion and wear resistance, without the fatigue resistance loss generally associated with hard coatings. The methanol environment accelerates crack nucleation and propagation, resulting in a sensible deterioration of the fatigue behaviour. A minimum soaking time seems to be necessary before the damaging effect of the environment begins. The coating has a certain protective effect against the environment, but this protection is insufficient for the specimen to achieve fatigue limits beyond those of the uncoated specimens. This deficiency can be related to small pores or defects in the coating, which allow contact between the substrate and the environment. Further tests are necessary to verify whether there exists a load under which the fatigue behaviour of the coated specimens is better than that of the uncoated specimens. Crack nucleation due to fatigue occurs close to the outer surface for all observed samples. For coated samples tested at the lowest stress level, crack nucleation seems to be located below the surface. This observation means that premature coating cracking, which characterises the nucleation mechanism at higher loads, did not occur at lower stress levels. The fracture surface of uncoated samples was clearly damaged by the aggressive solution, justifying the poor fatigue resistance.
Research limitations/implications
The obtained data do not represent actual S-N curves, which would necessitate a larger number of tests with proper statistics. Nevertheless, some indications of the DLC effects on 7075-T6 specimens in air and methanol environments can be deduced. The step-loading technique seems to be critical for tests in corrosive environments, probably because the total soaking time in the corrosive environment is generally higher than it is for the single-run test.
Originality/value
The originality of the paper lies in the application of the step-loading test procedure to quickly detect the mechanical and chemical driving forces that control the damage and structural integrity of light alloys components in very aggressive environments.
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Corrosion‐fatigue testing using precracked specimens has, in recent years, become an important means of evaluating structural alloys for service in corrosive environments. The…
Abstract
Corrosion‐fatigue testing using precracked specimens has, in recent years, become an important means of evaluating structural alloys for service in corrosive environments. The recent emphasis towards the use of precracked specimens for corrosion‐fatigue testing is based upon several factors. First, there is the general recognition that metallic structures of all types are prone to contain cracks and that the growth of such cracks can play a crucial role in overall structural performance; and secondly, a fracture mechanics technology basis has been developed for quantitatively assessing crack growth phenomena. The coexistence of a visible problem area and a means of attacking the problem has stimulated considerable activity in this field of endeavour.